The present invention relates generally to variable control of an electric motor to optimize its performance, and particularly to control of a brushless, permanent magnet motor utilized in a remote location, such as a downhole, wellbore environment.
Currently, motors, such as brushless, permanent magnet motors, have a rotor with permanent magnets rotatably mounted within a stator having a plurality of windings. The windings are sequentially energized to cause the rotor to rotate.
Generally, such motors use a direct couple rotor feedback position control. The rotor position or the BEMF of the unenergized coil is used to determine the exact rotor position. A position signal is then fed back to a controller which uses the position signal to sequentially energize the windings of the motor.
Presently, once the motor and motor control are designed, the position feedback signals cannot be changed during operation. Typically, the motor must be shut down, and the feedback sensor position relocated before the motor is restarted. This process is repeated until the best operating point can be determined.
In many applications, such as the remote operation of a submergible motor, this may not be possible. Submergible motors are used, for instance, in electric submergible pumping systems of the type utilized in pumping petroleum from wells beneath the surface of the earth. It is not feasible to remove the motor from the well in an effort to optimize the rotor position feedback signals for best performance.
The position of the rotor can be sensed in a variety of ways. For example, a plurality of Hall effect sensors can be used to sense the position of the rotor. The Hall effect sensors typically are mounted on a printed circuit board and fastened to the motor around the rotor. The Hall effect devices are located in this fixed position relative to the magnets used on the rotor and the windings of the stator. As the rotor magnets move past individual Hall effect sensors, a feedback signal or signals is provided to a control board which, in turn, uses these feedback signals to direct the firing of insulated gate bipolar transistors (IGBTs) to sequentially energize the windings of the stator.
However, because the position sensors typically are located at a single location along the axis of the rotor, the feedback signals may not optimize the performance of the motor when operating under load. For example, placing a load on the motor may cause a twisting of the drive shaft on which the rotor is mounted. This is particularly true with relatively long thin motors, such as those used in subterranean, wellbore environments. The twisting of the shafts, and thus the rotor, effectively causes suboptimal sequential energization of the stator windings relative to the rotor position.
It would be advantageous to be able to modify the position feedback signal in accordance with changes, such as shaft twisting, that occur during operation. It also would be advantageous to optimize the performance of the motor during real-time operation.
The present invention features an electric motor that comprises a stator and a rotor. The stator includes a plurality of windings, and the rotor is rotatably mounted in the stator. An energization system is coupled to the stator to sequentially energize the plurality of windings. A position sensor system is coupled to the rotor to sense rotor position. A processor is coupled to the energization system and to the position sensor system. The processor receives input signals from the position sensor system and outputs a control signal to the energization system based on the input signals. A feedback device generates a feedback control signal to the processor. The processor automatically adjusts the control signal to the energization system according to the feedback control signal.
According to another aspect of the invention, a method is provided for optimizing performance of a motor having a stator with a plurality of windings and a rotor rotatably mounted with respect to the stator. The method includes sequentially energizing the plurality of windings to rotate the rotor. The method further includes measuring the angular position of the rotor, and outputting a position feedback signal indicative of a position of the rotor, to a processor. The method further includes outputting a control feedback signal to the processor, and adjusting the position feedback signal according to the control feedback signal. The energization of the plurality of windings is controlled according to the adjusted position feedback signal.
According to another aspect of the invention, a method is provided for real-time adjustment of a motor to reduce the detrimental performance effects of a twisting of a motor shaft on which a rotor is mounted. The method includes measuring the angular position of the rotor at an axial position along the rotor, and measuring a parameter indicative of a degree of twisting in the shaft. The method further includes outputting a feedback signal corresponding to the angular position, and controlling energization of the motor based on the feedback signal and the measured parameter to compensate for the twisting of the rotor shaft.